The overall goal of this proposal is to determine the functions of Hrb, a novel ErbB3-binding protein identified using the intracellular portion of ErbB3 as bait in a yeast two-hybrid assay. ErbB3 is a member of the epidermal growth factor (EGF) receptor that are over-expressed in a variety of human cancers, and regulates cell growth and differentiation in response to specific growth factors such as heregulins. Our preliminary data demonstrate that Hrb suppresses the growth of cultured cells. Hrb mRNA levels vary with the cell cycle. We hypothesize that Hrb mediates activity of ErbB3 in growth regulation via a direct protein interaction. The specific aims are to: 1. Determine the role of Hrb in ErbB3 signaling in the different cultured cells; 2) Dissect functional domains of Hrb involved in ErbB3-binding and signaling (e.g., interaction with phosphoinositides) using truncated Hrb proteins with deletions in one or multiple domains and direct mutagenesis of Hrb; 3. Analyze Hrb-binding motif(s) of ErbB3 using directed mutagenesis of the intracellular portion of ErbB3. The proposed studies may contribute to understanding mechanisms underlying cancer cell growth, and to the ErbB-based chemo- or immuno-therapy of cancer.